The invention relates to an oxygen-depolarized electrode, in particular for use in chloralkali electrolysis, having a novel electrocatalyst coating and also an electrolysis apparatus. The invention further relates to a process for producing the oxygen-depolarized electrode and also its use in chloralkali electrolysis or fuel cell technology.
The invention proceeds from oxygen-depolarized electrodes which are known per se and are configured as gas diffusion electrodes and usually comprise an electrically conductive support and a gas diffusion layer having a catalytically active component. The invention proceeds in particular from oxygen-depolarized electrodes which comprise carbon modifications as electrocatalyst.
Oxygen-depolarized electrodes are a form of gas diffusion electrodes. Gas diffusion electrodes are electrodes in which the three states of matter, viz. solid, liquid and gaseous, are in contact with one another and the solid, electron-conducting catalyst catalyzes an electrochemical reaction between the liquid phase and the gaseous phase. The solid electrocatalyst is usually present in a porous film having a thickness in the range from about 200 μm to 500 μm.
Various proposals for operating the oxygen-depolarized electrodes in electrolysis cells of industrial size are known in principle from the prior art. The basic idea here is to replace the hydrogen-evolving cathode of the electrolysis (for example in chloralkali electrolysis) by the oxygen-depolarized electrode (cathode). An overview of the possible cell design and solutions may be found in the publication by Moussallem et al “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
The oxygen-depolarized electrode, hereinafter also referred to as ODE for short, has to meet a series of fundamental requirements in order to be usable in industrial electrolyzers. Thus, the electrocatalyst and all other materials used have to be chemically stable toward the alkali metal hydroxide solution used, e.g. a sodium hydroxide solution having a concentration of about 32% by weight, and toward pure oxygen at a temperature of typically 80-90° C. A high measure of mechanical stability is likewise required, so that the electrodes can be installed and operated in electrolyzers having an area of usually more than 2 m2 (industrial size). Further properties are: a high electrical conductivity, a high internal surface area and a high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and an appropriate pore structure for conduction of gas and electrolyte are likewise necessary, as is impermeability so that, for example, gas space and liquid space remain separated from one another in an electrolyzer. Long-term stability and low production costs are further particular requirements which an industrially usable oxygen-depolarized electrode has to meet.
A further development direction for utilization of the ODE technology in chloralkali electrolysis is to place the ion-exchange membrane which separates the anode space from the cathode space in the electrolysis cell directly on the ODE. A sodium hydroxide solution gap is not present in this arrangement. This arrangement is also referred to as zero gap arrangement in the prior art. This arrangement is usually also employed in fuel cell technology. A disadvantage here is that the sodium hydroxide solution formed has to be conveyed through the ODE to the gas side and subsequently flows downward on the ODE. Here, blockage of the pores in the ODE by the sodium hydroxide solution or crystallization of sodium hydroxide in the pores must not occur. It has been found that very high sodium hydroxide concentrations can arise here, as a result of which the ion-exchange membrane is not stable in the long term to these high concentrations (Lipp et al, J. Appl. Electrochem. 35 (2005)1015—Los Alamos National Laboratory “Peroxide formation during chlor-alkali electrolysis with carbon-based ODC”).
The use of inexpensive carbon materials such as carbon black or graphite as support material has to be avoided completely, in particular in the reduction of oxygen in an alkaline medium, because these generally promote the reaction according to reaction route (I) and thus lead to greatly reduced lives of the electrodes and to current yield losses (O. Ichinose et al. “Effect of silver catalyst on the activity and mechanism of a gas diffusion type oxygen cathode for chloralkali electrolysis”, Journal of Applied Electrochemistry 34: 55-59 (2004)).

A disadvantage of the reduction of oxygen in an alkaline medium, where “in an alkaline medium” means, for example, a concentrated, in particular 32% strength by weight, sodium hydroxide solution, using an electrocatalyst, e.g. a catalyst in which silver supported on carbon black is present, and a temperature in the range from 60° C. to 90° C. is that the hydrogen peroxide formed as an intermediate degrades the carbon of the carbon black, resulting in formation of cracks in the electrode and to mechanical instability of the electrode and the electrode becoming unusable. Due to this “carbon corrosion”, the supported electro catalyst likewise becomes detached from the support and the electrocatalyst thus becomes unusable.
It is likewise known (see O. Ichinose et al.) that the transfer of two electrons to oxygen can be avoided when silver is added to the carbon black; here, the step transferring four electrodes is preferred.
Similar effects also occur in the case of electrodes which are loaded with platinum and contain carbon black (L. Lipp “Peroxide formation in a zero-gap chlor-alkali cell with an oxygendepolarized cathode”, Journal of Applied Electrochemistry 35:1015-1024 (2005)). It is also disclosed that part of the hydrogen peroxide formed can be reduced further to the desired hydroxide ions by application of higher voltages and/or higher current densities. The possibility of the sequence of reactions according to reaction route (I) and (II) is thus described. However, since the reaction according to reaction route (I) takes place, the reaction according to reaction route IV likewise cannot be prevented, which in turn leads to a reduction in the yield of hydroxide ions. The process variants disclosed (see L. Lipp et al., O. Ichinose et al.) thus have the same economic and technical disadvantages.
Carbon nanotubes (CNTs) have been generally known to those skilled in the art at least since they were described in 1991 by Iijima 5 (S. Iijima, Nature 354, 56-58, 1991). Since then, the term carbon nanotubes has referred to cylindrical bodies comprising carbon and having a diameter in the range from 3 to 80 nm and a length which is a multiple of, at least 10 times, the diameter. Furthermore, these carbon nanotubes are characterized by layers of ordered carbon atoms, with the carbon nanotubes normally having a core which differs in terms of the morphology. Synonyms for carbon nanotubes are, for example, “carbon fibrils” or “hollow carbon fibers” or “carbon bamboos” or (in the case of rolled structures) “nanoscrolls” or “nanorolls”.
A further development in processes for the reduction of oxygen is the use of nitrogen-containing carbon modifications (P. Matter et al., “Oxygen reduction reaction activity and surface properties of nanostructured nitrogen-containing carbon”, Journal of Molecular Catalysis A: Chemical 264: 73-81 (2007). Here, catalytic activity for the reduction of oxygen is obtained by catalytic deposition of vapors of acetonitrile on support materials such as silicon dioxide, magnesium oxide which in turn contain iron, cobalt or nickel as catalytically active component. The process for the reduction of oxygen is characterized in that it is carried out in a 0.5 molar sulfuric acid solution.
WO 2010069490 A1 describes the use of nitrogen-modified carbon nanotubes (NCNTs) for the reduction of oxygen in an alkaline medium. Here, no noble metal-containing catalysts are used. However, experiments have shown that the NCNT-based electrodes do not have a satisfactory long-term stability.
DE102009058833 A1 describes a process for producing nitrogen-modified CNTs, with from 2 to 60% by weight of metal nanoparticles having an average particle size in the range from 1 to 10 nm being present on the surface of the NCNTs. A disadvantage here is that the production method is very complicated.
Various methods which can fundamentally be divided into wet processes and dry processes are known for producing gas diffusion electrodes. In the dry process, e.g. as described in DE102005023615A1, the catalyst is milled together with a polymer, frequently PTFE, to give a mixture and the mixture is subsequently applied to a mechanical support element, for example silver or nickel mesh. The powder is subsequently compacted together with the support to form an electrode by pressing. e.g. by means of a roller compacter.
In contrast, in the wet process, e.g. as described in EP2397578A2, a suspension of catalyst and polymer component is produced. This is applied to the support material and subsequently dried and sintered (I. Moussallem, J. Jörissen, U. Kunz, S. Pinnow, T. Turek, “Chlor-alkali electrolysis with oxygen depolarized cathodes: history, present status and future prospects”, J Appl. Electrochem. 2008, 38, 1177-1194).